The present disclosure relates to solar technology, especially to reflective solar collector technologies.
A trough-type solar energy collector typically includes a linear reflector, which often has a linear parabolic shape. The linear reflector can commonly rotate around its center of gravity to follow the path of the sun, reflecting the solar radiation onto a heat receiver. Although a heat receiver may have several shapes, but out of mechanical considerations, a heat receiver is most often in the form of a cylindrical tube that contains a fluid circulating inside to transfers heat collected by the heat receiver to an application area. In most cases, the cylindrical receiver tube is made of steel and has a spectral selective coating on its surface, The coating allows the linear receiver to absorb most of the solar radiation while significantly reducing thermal radiation loss.
Heat losses can be further reduced by enclosing the receiver tube by one or several glass envelops. The space between the receiver tube and the glass envelop can be sealed by vacuum, as in some commercially produced heat collecting elements (HCE), to further reduce heat loss. However, it is difficult to keep a vacuum bellow 13 Pa for a long period of time at temperatures of up to 400 degree C. Many failures with vacuum loss are due to hydrogen permeation as reported by NREL (the Parabolic Trough Workshop, Mar. 9, 2007, Golden, Colorado).
In trough-type solar energy collectors, increasing the operating temperature the receiver tubes can increase thermal cycle efficiency. Above a certain temperature, efficiency losses due to thermal radiation from the receiver tubes will increase more rapidly than the gains in thermal cycle efficiency. On the other hand, thermal radiation losses can be reduced by increasing concentration ratio. Current technology often uses a primary reflector comprising several reflective facets to focus maximum amount of solar radiation onto the receiver tube.
The parabolic reflector is often truncated due to economic and practical reasons, which decreases concentration ratio. The solar radiation reflected by the reflector cannot reach the side of the heat receiver facing the sun (and away from the reflector), which produces non-uniform heating of the receiver tube and can cause the receiver tube to bow and the glass envelop to break.
To increase the concentration ratio, several designs add a secondary reflector near the receiver tube. The receiver tube can be reduced in size. The solar radiation from the primary reflector that normally would have missed the receiver tube can now be reflected by the secondary reflector onto the non illuminated part of the receiver tube. The secondary reflector works like a light trap guiding spilled solar radiation to the non illuminated part of the receiver tube. Secondary reflectors can double the concentration ratio and allow higher optical tolerances.
FIGS. 1-5 show several trough collectors having secondary reflectors. FIG. 1 shows a trough-type solar collection system 100 (e.g. Duke Solar) comprising a primary parabolic reflector 110, a receiver tube 120, and a secondary reflector 130 positioned inside a vacuum envelop 140. There is no gap between the secondary reflector 130 and the receiver tube 120. The primary parabolic reflector 110 has a large curvature radius and low rim angle.
FIGS. 2 and 3 show trough-type solar collection systems 200 and 300 (reported by J. M. Gordon 1991 Solar energy, Volume 47, No. 6, pp 457-466). The trough-type solar collection system 200 has a primary reflector 210 having a high rim parabolic trough, an receiver tube 220, and a complex-shaped secondary reflector 230 positioned inside a glass tube 240. The trough-type solar collection system 300 has a primary reflector 310 having a parabolic trough, and a secondary reflector 320 comprising a compound parabolic collector (CPC) 322 and a flat receiver 323. The primary reflector 310 has a bigger curvature radius and a low rim angle.
FIGS. 4A-4D show various shapes of secondary reflectors 411-414 and their associated heat receivers 421-424 in parabolic trough collectors (reported by Harald Ries in Applied Optics, Volume 35, No, 13, May 1, 1996).
FIG. 5 shows a trough-type solar collection system 500 (reported by J. M. Gordon, 1993) that includes an ice-cream-cone-shaped CPC 510 and a cylindrical heat receiver 520 in a glass envelop 530 with a gap between the CPC 510 and the receiver tube 520. Gap losses are compensated by increased reflection area. This type of CPC is widely used in domestic hot water systems incorporating Dewar vacuum tubes.
Ideally, the top of secondary reflector should be in contact with the receiver tube to close the light trap. This is however problematic because the secondary reflector, often made of glass, cannot sustain the high thermal shocks when it touches the extremely hot receiver tube. Furthermore, non-uniform heating, as described above, causes the receiver tubes expand and bow, which makes it more difficult to keep the receiver tube apart from the secondary reflector. Leaving a space between the secondary reflector and the heat receiver produces an opening in the light trap and leads to losses in solar radiation.
Another problem associated with secondary reflectors is that solar rays are reflected several times before they strike the surface secondary reflector and then reaching the receiver tube. The increase number of reflections causes optical losses. For the above reasons, most trough-type solar collectors comprising secondary concentrators have lower efficiencies than trough collectors without secondary concentrators, which has prevented secondary reflectors in commercial trough collectors.